This invention relates to novel olefin polymer compositions which are resistant to fogging, especially when employed as a packaging film for moist products and as a greenhouse film for agricultural applications.
Agricultural films which are largely used in greenhouse culture or tunnel culture chiefly include soft ethylene resin films which are about 30 to 200 microns thick and which comprise, as a base resin, polyvinyl chloride (hereinafter abbreviated as PVC), branched low-density polyethylene (hereinafter abbreviated as LDPE), ethylene-vinyl acetate copolymers (hereinafter abbreviated as EVA), linear low-density polyethylene (hereinafter abbreviated as LLDPE), etc. Of the various properties required for the agricultural films, particularly important are weather resistance, anti-fogging properties, heat-retaining properties, and transparency. To cope with the recent situation confronting agriculture such as an increased cost and a shortage of labor, development of films having an extended duration of life before re-placement is desired.
The atmosphere within greenhouses or tunnels surrounded by an agricultural film is saturated with water vapor which evaporates from the soil or plants, and the water vapor dropwise condenses on the inner surface of a cold film to cause fogging. Water droplets on the film not only greatly reduce the incident sunlight due to irregular reflection but the droplets fall on the plants resulting in frequent occurrence of diseases.
To avoid fogging, antifog treatments on the inner surface of an agricultural film converting the film into an anti-stick water film and preventing adhesion of water droplets have been used thereby assuring transmission of sunlight into the greenhouse or tunnel, raising the soil temperature and air temperature within the greenhouse or tunnel, accelerating photosynthesis of the plants, accelerating healthy growth of the plants, and controlling the occurrence of plant diseases.
Currently employed antifog treatments include incorporation of an anti-fogging agent (antifog additive) into a film-forming ethylene resin compound and coating of an anti-fogging agent on a film.
However, an anti-fogging film obtained by coating a soft plastic film with an anti-fogging agent has not yet been employed practically as an agricultural film for the following reasons. Because of their low surface energy, soft plastic films for agricultural use generally have poor wettability and adhesion when coated with surface active agents or hydrophilic high polymeric substances which have been used as anti-fogging agents. This tendency is particularly conspicuous with soft ethylene resin films of low polarity, e.g., LDPE, EVA, and LLDPE films. Therefore, where an anti-fogging agent is spray coated with a power atomizer onto a soft ethylene resin film, the anti-fogging agent needs to be used in a large quantity and this increases cost, and a large amount of time is required for spray coating operation. Further, spray coating cannot be effected uniformly with insufficient anti-fogging effects arising. Where an anti-fogging agent is applied using a coater, etc., a large quantity of a coating is consumed, and the coating speed cannot be increased, resulting in an increase of cost. In either case, the coated anti-fogging agent is washed away together with running water droplets due to poor adhesion resulting in a very short life for the anti-fogging properties. Furthermore, the coated film undergoes blocking due to the stickiness of the anti-fogging agent. As a result, it has been impossible to retain anti-fogging effects in a stable manner for a long duration of at least 1 year, more desirably, several years. Most of the state-of-the-art agricultural films exhibit anti-fogging properties for a period of only about 1 month.
Anti-fogging agents commonly incorporated into the films include nonionic, anionic and cationic surface active agents.
Other methods for providing anti-fogging properties to agricultural films, in addition to the coating method and incorporation method, include chemical modification of the ethylene base resin or the ethylene resin film surface by introducing a polar group, such as a hydrophilic group. This technique, however, entails high cost at the present time and is difficult to apply to agricultural films.
On the other hand, coating type anti-fogging agents mainly comprising an inorganic hydrophilic colloidal substance and a hydrophilic organic compound have recently been developed as disclosed, e.g., in JP63-45432, JP63-45717, and JP64-2158. Suitable inorganic hydrophilic colloidal substances include colloidal silica, colloidal alumina, colloidal Fe(OH)2, colloidal Sn(OH)4, colloidal TiO2, colloidal BaSO4, and colloidal lithium silicate, with colloidal silica and colloidal alumina most generally used. Suitable hydrophilic organic compounds include various nonionic, anionic or cationic surface active agents; graft copolymers mainly comprising a hydroxyl-containing vinyl monomer unit and from 0.1 to 40% by weight of a carboxyl-containing vinyl monomer unit or a partial or complete neutralization product thereof; and sulfo-containing polyester resins.
Coating type anti-fogging agents used with this new film type have markedly improved wettability and adhesion and coated agricultural films exhibiting anti-fogging properties for a duration of about a half year to about 1 year are provided.
Under these circumstances, none of the state-of-the-art polyolefin-based agricultural films satisfies the requirements of long lasting anti-fogging properties.
In the field of packaging, whereby polymer films are used for covering or enclosing items of sale, and where film clarity or good optical properties are desired, there is a recognized need for polymer films which resist fogging when exposed to conditions of temperature and humidity which can produce condensation on the film. This condensation is especially troublesome when the interior of the package contains enough moisture to produce fogging of the interior surface of the film. Of special interest here is the packaging of food products, such as meats, bakery items, vegetables, fruits, and the like, though non-food items are also of interest.
Food products are often packaged in polymeric film for ease in handling by customers, to provide sanitary protection of the food product from the environment, to provide an easier and more economical wrapping than paper, or to provide a better view of the food product. Various polymeric materials such as polyvinyl chloride, poly(ethylene/vinyl acetate) and irradiated polyethylenes have been used in the food wrap market with plasticized polyvinyl chloride (PVC) being widely used in the market. Freshly cut meat and vegetables are often packaged in these films, and certain properties are desired of the film for it to perform satisfactorily as a food overwrap. Obviously, film used in this application should have good xe2x80x9csee throughxe2x80x9d clarity so the product can be viewed. High gloss is desired to add xe2x80x9csparklexe2x80x9d to the packages for aesthetic appeal to the customers. As these food products are packaged, they generally experience a temperature change from where they are packaged to where they are stored or sold. This temperature change can cause the formation of water droplets on the inside surface of the film (commonly called xe2x80x9cfogxe2x80x9d). These water droplets are usually objectionable because they hinder the view of the food products from the consumer and can cause spoilage, particularly to certain vegetables. To prevent water from condensing in droplets, an antifog agent is often added to the film. The antifog agents reduce the surface tension of the water and cause the water to form a continuous sheet, which is then transparent.
There is a perceived need in the market place for improved packaging films, especially for food products, where the widely used plasticized polyvinylchloride (PVC) requires replacement by a more acceptable film. Much of the PVC which has been used is plasticized with additives, e.g. dioctyl phthalate, which are becoming less acceptable in the market.
U.S. Pat. No. 4,189,420 discloses, inter alia, certain ethylene polymers blended with a polybutene and a mixed glyceride having at least one acyl group of 2 to 6 carbon atoms and at least one acyl group containing 8 to 22 carbon atoms.
U.S. Pat. No. 3,048,266 discloses, inter alia, an antifog agent of polyethylene oxide derivative in a polyolefin composition.
U.S. Pat. No. 3,048,263 discloses, inter alia, a polyolefin antifog agent comprising a monoglyceride of a fatty acid.
U.S. Pat. No. 2,462,331 discloses, inter alia, the incorporation into polyethylene of polyhydric alcohol esters or metal salts of either saturated or unsaturated monocarboxylic fatty acids.
U.S. Pat. No. 5,262,233 discloses agricultural films which may have incorporated therein an anti-fogging agent which may be a poly(ethylene oxide) of a long chain alcohol.
U.S. Pat. No. 4,486,552 discloses fog-resistant packaging films having incorporated therein (1) an alkoxylated alkyl phenol along with (or in combination with) a mixed mono-, di- and/or triglyceride, or (2) a polyoxyalkylene fatty acid ester, or (3) a combination of (2) and any part of (1) above.
U.S. Pat. No. 5,001,015 discloses polyolefin films with antistatic properties which include as possible antistatic agents the reaction products of polyalkoxylates with fatty alcohols.
Atmer(copyright) 502, is described in a September, 1998 data sheet as having long-lasting antifog properties in LDPE agricultural film. Atmer(copyright) 502 is a 2 mole ethoxylated stearyl alcohol, C18H37(OCH2CH2)2OH. Atmer(copyright) is a trademark of Uniqema.